1. Field of the Invention
The present invention relates to the field of miniaturized systems, and in particular to a method for producing a micromachined fluidic coupler for use in a miniaturized system.
2. Description of Prior Art
In recent years, there has been a great deal of interest and effort in the development of miniaturized chemical, electrochemical, and biological systems. One of the overall goals of this work is to develop an entire "system-on-a-chip" that performs sample preparation, sample transfer, sample analysis, and other related functions. Research teams at several locations have already developed many of the functional building blocks necessary for such a system, such as chemical sensors, valves, pumps, pressure sensors, etc.
Despite the development of these building blocks, a key limiting factor to successful systems integration has emerged: the lack of suitable microfluidic couplers for establishing fluidic connections in such systems. In order to assemble a system from discrete elements or simply to couple fluids into and out of a monolithic system, suitable microfluidic couplers are required.
The results of conventional methods for producing microfluidic couplers are shown in FIGS. 1-2. These conventional methods typically include the steps of etching an insertion channel 116 in the top surface of a substrate 102, bonding a cover 100 to the top surface, and inserting a capillary 114 into insertion channel 116. The etching step is typically performed using a conventional etching technique, such as crystal plane dependent etching, isotropic etching, or anisotropic dry etching.
Unfortunately, it is difficult to form an insertion channel having a correctly shaped cross section for receiving a capillary using these conventional etching techniques. Crystal plane dependent etching forms an insertion channel having a triangular cross section 104. Isotropic etching forms an insertion channel having a roughly semi-circular cross section 106. Anisotropic dry etching forms an insertion channel having a rectangular cross section 108. Because most capillaries are circular in cross section, they cannot be properly fitted to these insertion channels. Instead, an adhesive must be used to seal a gap 112 between capillary 114 and insertion channel 116, as shown in FIG. 2. Use of an adhesive to seal gap 112 hinders system performance and renders the connection between the capillary and insertion channel permanent rather than interchangeable.
An example of such a microfluidic coupler is described in Reay et al. "Microfabricated Electrochemical Detector for Capillary Electrophoresis", Proceedings of the Solid-State Sensor and Actuator Workshop, Hilton Head, S.C., Jun. 13-16, 1994, pp. 61-64. Reay describes the use of anisotropic dry etching to form a square or rectangular insertion channel in the top surface of a silicon substrate. A glass cover is then bonded to the substrate to seal the channel. Next, a capillary tube is inserted and sealed in the channel using an epoxy.
Another method for forming a microfluidic coupler includes the step of isotropically etching the top surfaces of two substrates to form in each substrate an approximately hemi-cylindrical channel. The two substrates are then bonded together with their respective channels aligned to form a somewhat cylindrical insertion channel. In practice, however, such channels are seldom perfectly cylindrical, and thus a good fit to the capillary is still difficult to achieve.
Another disadvantage of these conventional methods for producing microfluidic couplers is that they do not allow for the precise formation of subchannels in the substrate. To eliminate dead space in the microfluidic coupler, the insertion channel should terminate in a subchannel having a diameter that precisely matches the diameter of a bore of the capillary. Attempts to form matching subchannels using conventional etching techniques are generally unsuccessful. As a result, these conventional methods produce microfluidic couplers having geometric imperfections and potential dead spaces which can trap samples or reagents and disrupt fluid flow patterns.